1. Field of the Invention
The present invention relates to a process for the treatment of pulp mill effluents, and particularly to soda-containing effluents. The present invention, more specifically, relates to the ultrafiltration of such effluents.
2. Description of the Prior Art
The highly toxic nature of pulp or paper mill effluents is well known, as well as the highly deleterious results which follow the discharge of such effluents from a mill to a waterway. As these effluents are strongly polluting in nature, the art further recognizes the need for some treatment thereof prior to discharge.
As a norm, pulp mill effluents derived from paper pulp bleaching operations result from a plurality of chemical treatments or modifications in the pulping operations. Typically, the bleaching processes conventionally employed are comprised of a first chlorine treatment and a second sodium hydroxide treatment, usually followed by a water wash in order to remove compounds which have been rendered soluble in the process. Accordingly, chlorination liquors are generated from the washing of the pulp after the chlorine bleach, and soda treatment liquors are derived from the pulp washing after bleaching in sodium hydroxide. These chlorination liquors and, more especially, soda treatment liquors constitute the most abundant proportions of pollutants generated during these pulping operations.
Not only are pulping effluents highly toxic, conventional processes generated copious quantities thereof. For example, the typical manufacture of one metric ton of paper pulp generates about 30 m.sup.3 of chlorination liquors and about 10 m.sup.3 of soda treatment liquors, these quantities subject to some variation depending upon the precise nature of the pulp produced, as well as the specific manufacturing and bleaching processes employed.
The very nature of pulp mill effluents also varies within a range depending upon the quality of the wood employed in the pulping operation, e.g., deciduous versus resinous, as well as the delignification process employed for the extraction of the ligin component, in the form of ligno-sulfonates, which preceeds the bleaching of the pulp.
Because of the very complicated chemical nature of the pulping effluents, the polluting characteristics thereof are found to be rather complex and ill-defined. Pragmatically, it has been found convenient to characterize these effluents in terms of the biological oxygen demand (BOD), the chemical oxygen demand (COD), and color; these parameters having been accorded a status in the art in terms of defining the principle polluting characteristics of the subject effluents. Yet another method of identifying the degree of pollution attributable to pulping effluents consists of a determination of the organic carbon content of the effluent, i.e., carbon derived from organic molecules.
Diverse treatment processes for pulp mill effluents have been proposed, particularly treatments based upon the use of calcium hydroxide and active charcoal. More recently, the utility of various membranes, and apparatus for use in conjunction therewith, have been proposed for the treatment of pulp mill effluents. In this regard, see, Beder et al., Tappi, v. 53, n. 5, at 883-7, May, 1970, which describes a process for the treatment of paper mill effluents by reverse osmosis and, more especially, ultrafiltration.
Along these lines, there has become a recent recognition of a distinction between reverse osmosis and ultrafiltration, the latter characterized by the fact that the membranes retain only solute molecules larger than the solvent molecules, whereas reverse osmosis membranes will also retain solute molecules of a size on the order of magnitude as that of the solvent molecules.
While considerable attention has been paid to the utility of membrane processes, various significant difficulties have minimized the practical ability to employ the same, especially on a commercial scale. Among the most significant difficulties presented are: (1) purely reverse osmosis processes suffer the disadvantage of requiring the use of very high pressure since the hydraulic pressure must be greater than the osmotic pressure of the effluent; (2) the use of cellulose acetate membranes conventionally employed necessitates an acid pre-treatment to avert hydrolysis of the acetate, this pre-treatment requiring large quantities of acid due to the strong alkalinity of the soda treatment liquors, as well as generating considerable quantities of salts as undesirable ancillary byproducts should it be desirable to discharge the effluent into a river, or the like; (3) membranes conventionally employed often become clogged or loaded, leading to a reduction in the flow characteristics of the process; and, (4) ultrafiltration of these effluents is incapable of adequately eliminating pollution due to coloration.
Accordingly, the need exists to overcome the deficiencies of prior art processes directed to the treatment of pulp mill effluents, particularly those containing soda treatment liquors. Additionally, the need exists to provide a simple, economical, and yet industrially-efficient process for the treatment of pulp mill effluents.